Intended to be used with glow in the dark filament to give the Reylight Lan some added glow in the dark features. It can be installed between the head and body and the tail and body. With both, the light is still a complete circuit and will still turn on. If used between the tail switch and the body, a spacer stl is available to compensate for the distance created and allow the switch to have the same tension and not sink in too far.